1. Field of the Invention
This invention relates to compositions useful for inhibiting the deposition of scale and sludge deposits in aqueous systems particularly in steam generating boilers, and more specifically this invention relates to improved compositions comprising three or more additives selected from the group consisting of a) polyacrylic acid, copolymer of alkyl sulfonate and maleic anhydride, and mixtures thereof, b) amino (trimethylene phosphonic acid), hydroxyethylidene diphosphonic acid, diethylenetriamine pentaacetic acid, and mixtures thereof, and c) copolymer of styrene sulfonate and maleic anhydride, copolymer of acrylic acid and acrylamide, poly(acrylic acid- co-hypophosphite) and mixtures thereof, wherein at least one additive is selected from each of a), b) and c) supra.
2. Description of Prior Art
The formation of scale and sludge deposits on heating surfaces is the most serious problem encountered in boiler operations. External water softening only partially removes from the boiler feedwater those scale-forming substances. In most industrial boilers, internal chemical treatment is applied to the boiler water to combat the scale-forming tendency of entrained impurities, such as calcium, magnesium salts, iron and silica. The internal chemical treatment of boiler water can be accomplished by either non-precipitation or precipitation treatment programs. Under the non-precipitation treatment program, chelants such as NTA (nitrilotriacetic acid) and EDTA (ethylenediamine tetraacetic acid) are used. EDTA is capable of preventing calcium and magnesium deposits by solubilizing and moving these scale-forming substances through the boiler system until they are removed by blowdown. However, EDTA is known to be corrosive under certain conditions for example when there is excessive residual in the boiler.
Under the precipitation treatment program, commonly used precipitating chemicals include soda ash and sodium phosphates, e.g. disodium phosphate, trisodium phosphate and sodium metaphosphate. Thus, magnesium is precipitated as magnesium hydroxide by increasing the alkalinity of the boiler water. The precipitate or sludge, which can be made non-sticky and easily manageable by the addition of a dispersant or sometimes called sludge conditioner, is periodically or continuously removed from the boiler through blowdown. A wide variety of materials have been used as boiler dispersants such as sodium polyacrylates, sodium polymethacrylates, sodium carboxymethylcellulose, styrene-maleic anhydride copolymers, etc. However, the results of their dispersants have not been completely satisfactory.